Drive system abnormality detecting method and apparatus of an injection-molding machine

ABSTRACT

A drive system abnormality detecting apparatus of an injection-molding machine includes a microprocessor (19) for a controller, which causes a display unit (24) to indicate the occurrence of an abnormal situation. The drive of a servomotor (5) for an injection is stopped when the microprocessor concludes that the difference between the driving current of the servomotor and the output of a force detector, indicative of the resin pressure inside a cylinder (2), is not within predetermined permissible limits. The motor driving current and the detector output are individually periodically detected, during the time interval between the start of injection to the end of metering. By doing th is, the microprocessor prevents production of defective moldings, attributable to the occurrence of an abnormal situation in a screw drive system, and damage to components of the screw drive system and dies.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a drive system abnormality detectingapparatus of an injection-molding machine for detecting an abnormalsituation of resin pressure detecting means or screw drive means foraxially driving a screw of the injection-molding machine.

2. Background Art

In order to make a resin pressure, e.g., the resin pressure inside acylinder equal to a set resin pressure, in an injection-molding machine,the resin pressure inside the cylinder is indirectly measured by meansof resin pressure detecting means which is disposed at the rear end of ascrew. The measured resin pressure is compared with a screw injectionpressure set by means of injection pressure setting means. If the resinpressure inside the cylinder is lower than the set screw injectionpressure, the screw is driven in the direction to raise the resinpressure inside the cylinder. If the the resin pressure inside thecylinder is higher than the set screw injection pressure, on the otherhand, the screw is driven in the direction to lower the resin pressureinside the cylinder. Thus, closed loop drive control is effected.

According to the prior art described above, proper closed-loop controlcan be accomplished if both the resin pressure detecting means and thescrew drive means are in the normal state. If either the resin pressuredetecting means of the screw drive means malfunctions, however, properclosed-loop control cannot be achieved.

If the resin pressure detecting means becomes out of order, thusbecoming unable to measure the resin pressure inside the cylinder, forexample, it is impossible to keep the resin pressure inside the cylinderat the set resin pressure, and the repetition accuracy of pressurecontrol for the molding operation is considerably lowered, therebyinducing the occurrence of defective molding. Moreover, if the screwdrive means (hereinafter referred to as injection drive means) foraxially driving the screw is compulsorily driven in the direction toraise the resin pressure inside the cylinder, on the basis of a measuredvalue given by the resin pressure detecting means in trouble, theinjection drive means is subjected to overload, so that the screw, drivesystem, dies, etc. are adversely affected. In addition, the resinpressure inside the cylinder increases extraordinarily, thereby creatinga very dangerous situation.

In case of trouble in the injection drive means, moreover, the drivemeans cannot properly respond to feedback input despite the normalitythereof. Thus, the same problems as aforesaid are caused.

SUMMARY OF THE INVENTION

The object for the present invention is to provide a drive systemabnormality detecting apparatus of an injection-molding machine, capableof detecting an abnormal situation of resin pressure detecting means orinjection drive means without delay.

In order to achieve the above-mentioned object, the present inventioncomprises: resin pressure detecting means for detecting a resinpressure; driving force detecting means for detecting a driving force ofscrew drive means for driving a screw in an axial direction of thescrew; and abnormality detecting means operable to compare valuesdetected by the resin pressure detecting means and the driving forcedetecting means, and to detect an occurrence of an abnoraml situationwhen the difference between the compared values exceeds predeterminedpermissible limits.

According to the present invention, since an abnormal situation causedin the resin pressure detecting means or the injection axis drive meanscan be automatically detected, lowering of the repetition accuracy ofthe pressure control can be positively prevented, so that risks such asdefective molding, involved in unattended operation of theinjection-molding machine, are greatly reduced to ensure reasonablemolding operation. Moreover, since incomplete pressure control cannot becaused by the abnormal situation of the resin pressure detecting meansor the screw drive means, a screw, drive system, dies, etc. cannot bedamaged by extraordinary load, and resin cannot be subjected toextraordinary pressure, thus ensuring further improved safety of themolding operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the principal part of aninjection-molding machine mounted with a drive system abnormalitydetecting apparatus according to an embodiment of the present invention;and

FIG. 2 is a flow chart illustrating processes of abnormality detectionexecuted by means of the apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In an injection-molding machine of FIG. 1, a rotary motion of aservomotor 5 for an injection axis, for use as screw driving means, isconverted into a rectilinear motion of an injection drive shaft 4 bymeans of a transmission device 6, and a screw 1 disposed in a heatingcylinder 2 is caused to perform injecting operation by means of thedrive shaft 4.

Numeral 3 denotes a force detector for use as resin pressure detectingmeans which is disposed between the screw 1 and the injection driveshaft 4, and serves to indirectly measure the resin pressure inside thecylinder by detecting an axial pressure acting on the screw 1.

Numeral 17 denotes a microprocessor (hereinafter referred to as CPU) forNC, and numeral 19 denotes a CPU for a programmable controller(hereinafter referred to as PMC). The PMCCPU 19 is connected with a ROM23 which stores a sequence program for controlling the sequenceoperation of the injection-molding machine, etc. The PMCCPU 19 is alsoconnected with a RAM 20 which temporarily stores a value indicative ofthe resin pressure inside the cylinder, delivered from the forcedetector 3 through a pressure control unit 7 and digitized by means ofan A/D converter 8, and a driving current value delivered from aservo-circuit 10 to drive the servomotor 5 and digitized by means of anA/D converter 9, that is, a value indirectly indicative of a screwinjection pressure. Moreover, the NCCPU 17 is connected with a ROM 21which stores a control program for generally controlling theinjection-molding machine, and a RAM 12 for temporarily storing data.The NCCPU 17 is also connected, through a servo-interface 16, with theservo-circuit 10 which controls the drive of the servomotor for each ofvarious axes, such as the injection axis, mold clamp axis, screwrotating axis, ejector axis, etc. FIG. 1 shows only the servomotor 5 forthe injection axis, among other servomotors.

Numeral 13 denotes a nonvolatile common RAM 13 which, composed of abubble memory of CMOS memory, includes a memory section for storing anNC program for controlling various operations of the injection-moldingmachine and other programs, and another memory section for various setvalues and macro-variables. Numeral 18 denotes a bus arbiter controller(hereinafter referred to as BAC), which is bus-connected with the NCCPU17, the PMCCPU 19, the common RAM 13, an input circuit 15, and an outputcircuit 14. Buses used are controlled by means of the BAC 18. Further,the BAC 18 is connected, through an operator control panel (hereinafterreferred to as OPC) 22, with a manual data input device (hereinafterreferred to as CRT/MDI) 24 with a display unit for inputting the screwinjection pressure and various other data. The pressure control unit 7receives a command, which is obtained by converting a command pressuregiven by the output circuit 14 into an analog signal by means of a D/Aconverter 11, and compares the command pressure with the pressuredetected by means of the force detector 3. The difference between thesepressures is delivered to the servo-circuit 10, and a torque commandvalue delivered from the servo-circuit 10 to the servomotor 5 for theinjection axis, is clamped to effect torque limiting operation.

In the arrangement described above, the injection-molding machineperforms a molding operation, based on the sequence control by thePMCCPU 19 and the control of various operations by the NCCPU 17.

Referring now to FIG. 2, there will be described the operation of theinjection-molding machine according to this embodiment, includingabnormality detection executed by the PMCCPU 19 at intervals of apredetermined period, during processes from the start of injection tothe end of metering.

When the injection is started, a programmed injection pressure (maximumpressure is outputted if there is no injection pressure command at thetime of injection) is delivered from the PMCCPU 19 to the pressurecontrol unit 7, through the BAC 18, output circuit 14, and D/A converter11, and is compared with the pressure detected by means of the forcedetector 3. In accordance with the difference between these pressures,torque limit means in the servo-circuit 10 performs the torque limitingoperation.

Meanwhile, the NCCPU 17 starts pulse distribution to the servo-circuit10 for the injection axis through the servo-interface 16, in accordancewith an injection start command, to drive the servomotor 5 for theinjection axis. When a pressure hold process is started after theinjection process is finished, a command pressure for each pressure holdstep is delivered to the pressure control unit 7 in the same manner asaforesaid. In accordance with the difference between the commandpressure and the pressure detected by means of the force detector 3, thetorque limit means in the servo-circuit 10 performs a torque limitoperation. Thus, hold pressure control is achieved so that the commandpressure and the detected pressure are equal.

In the meantime, the pressure detected by means of the force detector 3is supplied to the A/D converter 8 through the pressure control unit 7.A detected pressure Pp, a digital signal obtained as a result of theconversion by means of the converter 8, is written in the RAM 20 atintervals of a predetermined sampling period. Also, a driving currentvalue Ps of the servomotor 5 for the injection axis is converted bymeans of the A/D converter 9, and is written in the RAM 20 insynchronism with the sampling period of the pressure Pp.

During the time interval between the start of injection and the end ofmetering, the PMCCPU 19 performs abnormality detection shown in FIG. 2at intervals of a predetermined period.

First, the value Pp, indicative of the resin pressure inside thecylinder detected by means of the force detector 3, and the value Ps,indicative of the driving current value of the servomotor 5 orindirectly indicative of the screw injection pressure, are read out fromthe RAM 20 (step S1). Then, the difference between the value Ppindicative of the resin pressure inside the cylinder and the value Psindicative of the screw injection pressure is obtained, and whether thisdifference is within predetermined permissible limit ε is determined(step S2). If the difference between the resin pressure Pp inside thecylinder and the screw injection pressure Ps is within the predeterminedpermissible limits ε, the force detector 3 and the servomotor 5 areregarded as normal, and the program proceeds to step S3. Thereupon,whether a metering end signal is already inputted, that is, whether thecycle processes from the start of injection to the end of metering arefinished, is determined. If the metering is not finished, the programreturns to step S1, whereupon a monitor loop consisting of steps S1, S2,and S3 is executed to continue monitoring the resin pressure Pp insidethe cylinder and the screw injection pressure Ps.

Meanwhile, if either the force detector 3 or the servomotor 5 for theinjection axis, for use as the screw drive means, malfunctions so thatthe force detector 3 cannot normally detect the resin pressure insidethe cylinder or that abnormal current flows through the servomotor 5,during the execution of the monitor loop, the difference between theresin pressure Pp inside the cylinder and the screw injection pressurePs, which should be substantially equal, increases and exceeds thepredetermined permissible limit ε. This situation is detected in stepS2. In this case, the program for the operation of the PMCCPU 19proceeds to step S4, whereupon the CPU 19 delivers an alarm to theCRT/MDI 24 through the BAC 18 and the OPC 22. Thus, the CRT/MDI 24 makesan abnormality indication, thereby warning the operator that the forcedetector 3 or the servomotor 5 is out of order. Further, the driveoutput of the servomotor 5 is stopped, thereby preventing an accidentwherein the resin pressure inside the cylinder increases extraordinarily(step S5).

In this embodiment, the force detector 3, which is disposed between thescrew 1 and the injection drive shaft 4, and adapted to detect the resinpressure acting in the axial direction of the screw 1, therebyindirectly measuring the resin pressure inside the cylinder, is used asthe resin pressure detecting means for detecting the resin pressure.However, the detecting means may be of any construction, provided it candetect the resin pressure, whether directly or indirectly. For example,a presure sensor inside the cylinder 2 may be used for this purpose. Inthe aforementioned embodiment, moreover, the output of the forcedetector 3 and the driving current of the servomotor 5 for the injectionaxis are converted into digital signals for comparison. Alternatively,however, the difference between the output of the force detector 3 andthe driving current of the servomotor 5 for the injection axis may bedetected by means of a differential amplifier or the like so that anabnormality detection signal is outputted to indicate the occurrence ofan abnormal situation when the difference exceeds a predetermined level.

What is claimed is:
 1. A method for detecting a malfunction in operationof an injection-molding machine having a driver for a screw, comprisingthe steps of:detecting a resin pressure in the injection-molding machineand generating a first pressure value; detecting a driving force of thedriver for driving the screw in the axial direction of the screw, andgenerating a second pressure value; and comparing the first and secondpressure values, and detecting a malfunction in the injection-moldingmachine when a difference between the compared first and second pressurevalues exceeds a predetermined value.
 2. A drive system abnormalitydetecting apparatus of an injection-molding machine having screw drivemeans for driving a screw in an axial direction of the screw,comprising:resin pressure detecting means for detecting a resin pressurein said injection-molding machine and providing a first pressure value;driving force detecting means for detecting a driving force of the screwdrive means for driving the screw in the axial direction of the screwand for providing a second pressure value; and abnormality detectingmeans for comparing the first pressure value provided by said resinpressure detecting means and the second pressure value provided by saiddriving force detecting means, and for detecting an occurrence of anabnormal situation when a difference between the compared first andsecond pressure values exceeds predetermined permissible limits
 3. Adrive system abnormality detecting apparatus of an injection-moldingmachine according to claim 2, wherein the screw drive means has aservomotor as a drive source therefor, and wherein said driving forcedetecting means is operable to detect a driving current value of saidservomotor which is indicative of the driving force of the screw drivemeans.
 4. A drive system abnormality detecting apparatus of aninjection-molding machine according to claim 2, wherein theinjection-molding machine includes a cylinder accommodating the screwtherein, and wherein said resin pressure detecting means is operable todetect the resin pressure inside the cylinder.
 5. A drive systemabnormality detecting apparatus of an injection-molding machineaccording to claim 3, wherein the injection-molding machine includes acylinder accommodating the screw therein, and wherein said resinpressure detecting means is operable to detect the resin pressure insidethe cylinder.
 6. A drive system abnormality detecting apparatus for aninjection-molding machine having a drive for a screw, comprising:apressure detector for detecting axial pressure acting on the screw andfor generating a first pressure value; a driving force detector fordetecting a driving force of the screw in the axial direction and forproviding a second pressure value; and means, coupled to said pressuredetector and said driving force detector, for comparing the first andsecond pressure values, and for detecting a malfunction in operation ofthe injection-molding machine when a difference between the comparedfirst and second pressure values exceeds a predetermined value.